Semi-permeable membranes have a long history of use in separating components of a solution. Such membranes are a type of filter able to retain certain substances while transmitting others. The components of the feed fluid that pass through the membrane are the “permeate” and those that do not pass through the membrane (i.e., are rejected by the membrane or are held by the membrane) are the “retentate”. In practice, the permeate, the retentate, or both streams may represent the desired product and may be used as obtained or may be subjected to further processing. In order to be economically viable, the membrane must provide sufficient flux (the rate of permeate flow per unit of membrane area) and separation (the ability of the membrane to retain certain components while transmitting others).
The degree of separation and permeate flux obtained in a membrane process are determined in large part by the general morphology of the membrane together with its physio-chemistry. Utilizing established membrane formation techniques, a given polymer type can be used to fabricate a wide variety of membranes including those with relatively large pores (e.g., microfiltration), those with smaller pores (e.g., ultrafiltration), or even those with pores sufficiently small that solute transport through the membrane is governed by the interactions among specific chemical functional groups in the membrane polymer and the feed components (e.g., nanofiltration (NF), reverse osmosis (RO), gas separation, pervaporation).
Semi-permeable membranes can be described by several different classifications. One method of classifying liquid permeating membranes is to represent them as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), or reverse osmosis (RO). These classes are not based on any single exact, formal definition, but are used in general terms in the membrane industry.
Microfiltration membranes are those membranes with pores greater than about 0.1 microns. The upper pore size limitation of the microfiltration membranes is not well defined, but can be considered to be about 10 microns. Materials with pore sizes larger than about 10 microns are generally not referred to as membranes. Microfiltration membranes are commonly used to retain small particulates and microbes. Typically, these membranes permeate smaller components, such as, simple salts and dissolved organic materials having a molecular weight of less than about 100,000 grams per mole. Microfiltration membranes usually possess the highest water permeability of the four classes of membranes, due to their large pore diameters as well as their typical high pore density. The pure water permeability (A value) of these membranes is commonly greater than about 5,000.
Ultrafiltration membranes typically are characterized by pore sizes of from about 0.1 micron to about 1 nanometer. Ultrafiltration membranes are commonly classified by their ability to retain specific sized components dissolved in a solution. This is referred to as the molecular weight cut-off (MWCO). Ultrafiltration membranes are commonly used to retain proteins, starches, and other relatively large dissolved materials while permeating simple salts and smaller dissolved organic compounds. The water permeability of ultrafiltration membranes is commonly in the range of from about A=100 to about A=5000.
Nanofiltration membranes typically are defined as membranes which possess the ability to fractionate small compounds (i.e., those with molecular weights less than 1000). The small compounds are often salts, and nanofiltration membranes are commonly used to permeate monovalent ions while retaining divalent ions. Nanofiltration membranes typically posses ionized or ionizable groups. Although not wishing to be bound by theory, it is believed that the nanofilters can affect the separation of ionic materials through a charge-based interaction mechanism. Nanofiltration membranes also can be used to separate uncharged organic compounds, sometimes in solvents other than water. The water permeability of nanofiltration membranes is commonly in the range of from about A=5 to about A=50.
Reverse osmosis membranes can retain all components other than the permeating solvent (usually water). Like nanofiltration membranes, reverse osmosis membranes can contain ionic functional groups. Reverse osmosis membranes are commonly used to remove salt from water and concentrate small organic compounds. The water permeability of reverse osmosis membranes is commonly in the range of from about A=2 to about A=20.
Although the mechanisms that govern membrane performance are not exactly defined, some basic theories have been postulated. A good review of some membrane transport theories can be found in, The Solution Diffusion Model: A Review, J. G. Wijmans, R. W. Baker, Journal of Membrane Science, 1995, vol 107, pages 1–21.
It is generally believed that microfiltration and ultrafiltration operate via a pore flow model where the pores of the membrane sieve the components of the feed solution through primarily physical interaction. Chemical interactions between the chemical functional groups on the pore wall and the chemical functional groups of the feed solutions are believed to often play only a minor role in governing separation by microfiltration and ultrafiltration membranes.
In nanofiltration and reverse osmosis membranes, the general belief is that these membranes affect separation through both physical and chemical interactions. Since the pore sizes of these membranes are so small, thought by some to be simply the void space between atoms or chains of atoms, large particles are retained by these membranes because they are physically too large to pass through the membranes. The transport of small components is thought to be governed in part by size-based sieving, as with MF and UF membranes, but also is influenced by interactions between the membrane material and the solute. An NF membrane having an abundance of negatively charged functional groups, for example, will tend to preferentially retain multivalent anions over multivalent cations due to charge repulsion (while maintaining charge neutrality). A membrane with a net positive charge will tend to retain multivalent cations over multivalent anions.
Membranes have also been used in other applications such as pervaporation and gas separation. Typically, in these applications, the membranes permeate gaseous and not liquid materials. Some membranes used in reverse osmosis and nanofiltration have been found to function suitably in pervaporation and gas separation.
In addition, semi-permeable membranes also can be classified by their structure. Examples are symmetric, asymmetric, and composite membranes. Symmetric membranes are characterized by having a homogeneous pore structure throughout the membrane material. Examples of symmetric membranes are some microfiltration membranes, many ceramic membranes, and track-etched microporous membranes.
Asymmetric membranes are characterized by a heterogeneous pore structure throughout the membrane material. These membranes usually posses a thin “skin” layer having a smaller pore structure than the underlying material. Most commercially available ultrafiltration membranes posses an asymmetric structure.
Composite membranes are defined as having at least one thin film (matrix) layered on a porous support membrane. The porous support membrane is commonly a polymeric ultrafiltration or microfiltration membrane. The thin film is usually a polymer of a thickness of less than about 1 micron.
While many types of separations involving a wide range of feed solutions have been made possible through the use of semi-permeable membranes, some feed solutions contain substances that cause the degradation of the membrane or membrane performance and render the membranes impractical for separation of these feed solutions. A decline in performance can be caused by alterations in the morphology and/or the physio-chemical integrity of the membrane. For example, a feed solution can include substances that interact with membrane components to plasticize, dissolve or react with them chemically thus degrading their structure and/or function. Generally, solvents are examples of substances that can plasticize or dissolve membrane components. Examples of substances that may degrade membrane components include acids, bases, oxidants and the like.
The chemical mechanism of action of acids on various chemical functional groups is well known. Although not wishing to be bound by theory, it is believed that the most useful definitions and descriptions of an acid are those referred to as a Lewis acid or a Bronstead acid. A Lewis acid is a compound that is capable of accepting electrons. The more colloquial usage of the term “acid” is that of a Bronstead acid, compounds that can donate protons. Bronsted acids all exhibit Lewis acidity because the proton of a Bronstead acid is capable of accepting electrons. Examples of Bronstead acids include acids such as, for example, sulfuric acid, phosphoric acid, nitric acid, hydrochloric acid, and acetic acid. Similarly, examples of Lewis acids include boron trifluoride, aluminum trichloride, and iron trichloride.
Both Lewis and Bronstead acids are capable of promoting polymer degradations. In aqueous media, this process is often referred to as acid hydrolysis.
When acids attack the polymers of a semi-permeable membrane, the degradation often is observed as an increase in permeate flow through the membrane, a decrease in solute rejection by the membrane, or a combination of a changes in both of these performance properties. Significant changes in either of these properties can make the use of a membrane for separation impractical. Commonly, this type of performance degradation is observed when commercial polyamide nanofiltration (NF) and reverse osmosis (RO) membranes are utilized to process strongly acidic feeds. Although initially their performance may be sufficient to perform the desired separation, the performance rapidly deteriorates, i.e., the membranes lose the ability to retain dissolved metals, such as, cations and/or organic compounds in a short period of time.
Polymeric membranes with stability toward acids are known. Examples of polymers that are relatively stable towards acids and can be used to prepare membranes include polyolefins such as, for example, polyethylene and polypropylene, polyvinylidene flouride, polysulfones, polyethersulfone, and polyether ketones. However, when these polymers are used in a dense film capable of retaining a high degree of dissolved metal cations, and organic compounds, they are unable to permeate acids effectively. Conversely, when these polymers are used to form more porous, less dense morphologies, the resulting polymeric membranes can transmit a high degree of the dissolved acids, but then the membranes are unable to effectively separate dissolved metal cations and organic compounds. Although not wishing to be bound by a theory or mechanism, it is believed that the ineffectiveness of these polymeric membranes is due to the general lack of suitable chemical functional groups which act in a discriminating fashion toward the transport of one feed chemical species as compared with another.
Controlling the deleterious action of acids on semi-permeable membranes is of particular interest because of the numerous acid containing feeds (acid feeds) which otherwise could be treated by membrane filtration. As a result of their ability to dissolve, degrade, and render compounds soluble, acids are often the chemical of choice for various extractions, cleaning processes, and numerous other applications.
Conventional technology available for treatment of acid feeds is inefficient due to the loss of valuable extracted components as well as to the costs associated with environmental waste treatment and disposal of contaminated acids. In addition, there is a continuing need for separation media and/or techniques that, among other things, allow efficient recovery of valuable components from acid feed streams and/or recovery of acids for recycle use.
There is a lack of semi-permeable membranes capable of removing dissolved metal cations, and organic compounds from liquid-based feed streams while possessing exceptional stability and permeability to acids. In one aspect, the present invention provides membranes suitable for such uses.